Cholera toxin - a foe & a friend

Abstract

After De΄s pivotal demonstration in 1959 of a diarrhoeogenic exo-enterotoxin in cell-free culture filtrates from Vibrio cholerae (of classical biotype), much insight has been gained about cholera toxin (CT), which is arguably now the best known of all microbial toxins. The subunit structure and function of CT, its receptor (the GM1 ganglioside), and its effects on the cyclic AMP system and on intestinal secretion were defined in the 1970s, and the essential aspects of the genetic organization in the 1980s. Recent findings have generated additional perspectives. The 3D-crystal structure of CT has been established, the CT-encoding operon has been shown to be carried by a non-lytic bacteriophage, and in depth knowledge has been gained on how the bacterium controls CT gene expression in response to cell density and various environmental signals. The mode of entry into target cells and the intracellular transport of CT are becoming clearer. CT has become the prototype enterotoxin and a widely used tool for elucidating important aspects of cell biology and physiology, e.g., cell membrane receptors, the cyclic AMP system, G proteins, as well as normal and pathological ion transport mechanisms. In immunology, CT has emerged as a potent, widely used experimental adjuvant, and the strong oral-mucosal immunogenicity of the non-toxic B-subunit (CTB) has led to the use of CTB as a protective antigen together with killed vibrios in a widely licensed oral cholera vaccine. CTB has also been shown to promote immunological tolerance against certain types of mucosally co-administered antigens, preferably tissue antigens linked to the CTB molecule; this has stimulated research and development to use CTB in this context for treatment of autoimmune and allergic diseases. In summary, in the 50 years after De΄s discovery of CT, this molecule has emerged from being the cholera patient΄s "foe" to also becoming a highly useful scientist΄s "friend".

Fig. 1

Crystallographic structure of cholera toxin (a), its A (b) and B-subunits (c). In (d) the position of the residues in CTB differing between pre-1993 El Tor and Classical CTs are highlighted.

Fig. 2

Diagrammatic representation of cholera toxin gene regulation.

Fig. 3

Cholera toxin B-subunit GM1 receptor binding pocket The pentasaccharide structure of the GM1 receptor is shown (Gal, stands for Galactose, GalNac for N-acetylglucosamine; NAN for N-acetylneuraminic acid and Glc for Glucose).The Gal and NAN sugar residues in green and red are those that establish direct interactions with the B-subunit, either directly or via the solvent (small spheres) Interactions, mostly hydrogen bonds, are depicted by broken lines. The amino acid residues of CTB found to be indispensable for binding are in bold and larger font. The asterisk specifically denotes the amino acid residue (Gly33) that comes from the adjacent B subunit. All indicated interactions involve side chains of amino acids except for those shown in italics. (Lengths of broken lines are not meant to depict real atomic distances; also, relative locations of amino acids are merely diagrammatic).

Fig. 4

Cholera toxin intracellular traffic.

Fig. 5

Adjuvant-active proteins based on CT and/or LT enterotoxins. (a) Amino acid changes and deletions in the A subunits described to result in adjuvant active toxins with no or much reduced enterotoxicity. The original amino acid is indicated, followed by its position along the mature A-subunit polypeptide sequence and then mutated amino acid. If the amino acid was deleted, this is indicated in parenthesis (del); the type of mutated toxin (CT or LT) is also shown in parenthesis. (b) A different type of adjuvant-active molecule based on a fusion protein between CTA1 and the immunoglobulin-binding protein DD (a derivative of Staphylococcus aureus protein A). (c) Adjuvant-active complex obtained by coupling TLR-9 activating CpG oligonucleotides (shown as heptagons) to CTB (coloured yellow) pentamers.